1. Field of the Invention
The present invention relates to an electrostatic actuator which moves a movable element by an electro-static force applied between a fixed substrate and the movable element, and more particularly to an electret actuator which uses an electret for the movable element.
2. Description of the Related Art
As an actuator, an electromagnetic actuator that uses an electromagnetic force has conventionally been used frequently. Such an electromagnetic actuator is configured by using an electromagnetic coil or a permanent magnet. Thus, not only the structure becomes complex but also power consumption becomes large during driving.
On the other hand, the electrostatic actuator is not only simple in configuration but also can be driven with low consumption, and thus it is now the focus of attention as an actuator suited to miniaturization.
The electrostatic actuator conveys the movable element by the horizontal component (driving force) of an electrostatic force applied between driving electrodes installed in the fixed substrate and the movable element arranged on the fixed substrate. During the driving, the driving force is increased as the electrostatic force becomes larger. Simultaneously, however, adsorbability of bonding the movable element to the fixed substrate is increased to enlarge the friction force, consequently blocking movement of the movable element relative to the fixed substrate. Accordingly, measures must be taken to improve driving efficiency.
To solve the problem of the adsorption of the movable element, there has been proposed a conveying device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-215853. The conveying device comprises a fixed substrate which has strip-shaped driving electrodes extended in a direction orthogonal to the conveying direction of a movable element, and the movable element which has a resistor. In this case, the every two strip-shaped driving electrodes are connected to constitute three-phase electrode sections. The electrode sections are arranged at equal pitch.
According to this conveying device, first, in an initial polarization step, an initial polarization pattern (−, +, +) is applied to the electrode sections with respect to the movable element set in an operation start position. Then, the initial polarization pattern and the movable element are subjected to electrostatic polarization, and the movable element is charged by patterns (+, −, −) reverse to the initial polarization patterns. Next, driving patterns are applied to the electrode sections in order of (+, −, 0)→(0, +, −)→(−, 0, +). Then, the movable element is moved relative to the fixed substrate by attraction and repulsion of an electrostatic force generated by the charging patterns of the movable element and the driving patterns applied to the electrode sections.
In this case, when initial polarization occurs, adsorption is applied by the initial polarization and charging patterns of each electrode section, and an operation is prevented even when the driving patterns are applied in next steps. Thus, Publication No. 11-215853 proposes a driving method of preventing adsorption by setting a separation step after the initial polarization step.
That is, according to the driving method, in a state in which the initial polarization pattern is applied to the electrode sections of the driving electrode and the charging pattern is formed in the movable element, to release bonding between the movable element and the fixed substrate, separation pattern (+, −, −) of polarities reverse to the initial polarization pattern (−, +, +) is applied to the electrode sections. As the movable element thereby floats, an operation is facilitated by subsequently applying the driving pattern to the electrode sections. Then, by inserting the separation pattern before each driving pattern, it is possible to realize stable driving of reduced adsorption.